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Published on: May 8, 2018
Confidence limit variation for a single IMRT system following the TG119 protocol
J D Gordon1, S P Krafft, S Jang
1Department of Radiation Oncology, The University of Arizona, Tucson, Arizona 85724, USA. jdgordon@email.arizona.edu
Purpose:
To evaluate the robustness of TG119-based quality assurance metrics for an IMRT system.
Methods:
Four planners constructed treatment plans for the five IMRT test cases described in TG119. All plans were delivered to a 30 cm x 30 cm x 15 cm solid water phantom in one treatment session in order to minimize session-dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized film and an ionization chamber. Per-field measurements were collected using a diode array device at an effective depth of 5 cm. All data collected were analyzed using the TG119 specifications to determine the confidence limit values for each planner separately and then compared.
Results:
The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. The resulting confidence limits were 3.13%, 1.98%, 3.65%, and 4.39%. Confidence limit values determined by composite film analysis were 8.06%, 13.4%, 9.30%, and 16.5%. Confidence limits from per-field measurements were 1.55%, 0.00%, 0.00%, and 2.89%.
Conclusions:
For a single IMRT system, the accuracy assessment provided by TG119-based quality assurance metrics showed significant variations in the confidence limits between planners across all composite and per-field evaluations. This observed variation is likely due to the different levels of modulation between each planner's set of plans. Performing the TG119 evaluation using plans produced by a single planner may not provide an adequate estimation of IMRT system accuracy.
